I. Field of the Invention
In some embodiments, the present invention relates to novel methods of using hydrogen in Cr/silica-based olefin polymerization processes to modulate the properties of the resulting polyolefin polymer. For example, such methods may be used to make polymers having a broad and/or a bimodal distribution of molecular weights.
II. Description of Related Art
By broadening the molecular weight distribution of a polyolefin polymer improved physical and rheological properties can be obtained. However, the production of such polymers can be difficult. Attempts to obtain such polymers by mixing two or more sets of polymer molecules, each having a different and distinct average molecular weight, typically result in compositions that do not act like a homogenous mixture of the two or more polymers because the mixing process does not go to completion and small pockets of each polymer were retained. Examples of attempts to mix polymers in this manner are described in U.S. Pat. Nos. 3,592,880, 4,352,915, and 4,357,448.
Other attempts to address this problem include using two reactors in series to produce different polymers in a continuous process is described in European patent 0580930A1 and PCT Patent Publication WO 1995/011930. Lack of flexibility is one of the downsides of these processes. Typically, the processes require the production of the higher MW polymer in the first reactor because they require the use of a low concentration of hydrogen in the first reaction vessel and a higher concentration of hydrogen in the second reaction vessel. Furthermore, neither of these patents provides an example of using a chromium/silica catalyst to perform such a two reactor process. PCT Patent Publication WO 1995/011930 notes that the preferred method of controlling the molecular weight with a chromium catalyst is through changing the polymerization temperature rather than modifying the presence and concentrations of hydrogen gas. Additionally, the method described in both publications requires that if a comonomer is used, the comonomer is preferentially used in the first reactor vessel. Using a comonomer in only the first reactor vessel limits the types and properties of the polymers that can be produced in the second reaction vessel.
In U.S. Pat. No. 6,063,878, attempts were made to address these difficulties by utilizing other factors such as ethylene concentration and temperature to control the average molecular weight of the polymers produced in the loop system. While a broad or bimodal distribution of molecular weight polymers could be obtained, polymer properties could only be controlled in to a limited extent.
In U.S. Pat. No. 7,034,092, a bimodal dual reactor polymerization system preferably using a Ziegler-Natta catalyst or a metallocene catalyst with hydrogen gas was presented, but the method requires greater than 30 wt % of solids compared to the diluent in the first reactor in order to have sufficient control over the properties of the bimodal product. This particular polymerization methodology, in order to obtain the effective ratio of solids to diluent, requires the use of high, specific amounts of ethylene compared to diluent which can limit the types of polymers made with the process.
As such, the development of more advantageous processes to make homogenous polyolefins that have a broad and/or bimodal distribution of molecular weights is desirable.